A factory manufactured 2620 ‘Small’, ‘Medium’ and ‘Large’ size T-shirts. The ratio of the number of ‘Medium’ size T-shirts to the number of ‘Small’ size T-shirts was 1 : 3. After of the ‘Small’ size T-shirts, of the ‘Large’ size T-shirts and none of the ‘Medium’ size T-shirts were sold, there were 1645 T-shirts left. How many ‘Small’ size T-shirts were there at first?